Acids, acid strength hard, definition

Concepts of acidity and basicity are, in practice, defined and evaluated by their utility. Since overly formd definitions can be restrictive the concepts of acidity evolve towaids more comprehensive definitions. For example the Lewis definition includes the Broensted definition simply regarding the proton as an electron acceptor. Because the interaction of Broensted acids and bases in solutions involves a common process, protic transfer, scales of acidity can be established, for example the Hammett [1] acidity function. For Lewis acid-base interaction there is no common process to provide a unique basis for comparisons of acid strength. Experimentally, the strength of a Lewis acid depends upon the particular Lewis base. The classification of acids and bases as hard or soft in the principle of hard and soft acids and bases (HSAB principle) clarifies the interactions of Lewis acids and bases [2a]. Strong interactions occur between hard acid and hard base, or between soft acid and soft base, hi the hard-hard interaction there is a considerable electrostatic contribution to bonding and in the soft-soft interaction there is a major covalent contribution to bonding. The use of density functional analysis has clarified the concepts of hardness and softness and an empirical ranking of Lewis acids, based on local hardness is, proposed [2c]. 

Stability constant comparisons should be made with some caution, since the accuracies of some values may be no better than 1, and experimental conditions (ionic strengths and the salts used to maintain them) are not identical for all systems. Nonetheless, the Table 3 values give some idea of relative stabilities for a range of ligands. The order of logmA i values for the halide ions should be noted, for comparison of this set of values for Zn + with analogous values for Cd + and Hg + (Table 4) positions these cations according to the Hard Soft Acids and Bases principle, with Zn + just on the hard side, Cd + just on the soft side, of the hard/soft border, and Hg + definitely soft. 

Definition of hard/soft character is the result of empirical observations and trends in measured stability of complexes. For example, hard acids (such as Fe3+) tend to bind the halides in the order of complex strength of F > Cr > Br > I, and soft acids (such as Hg2+) in the reverse order of stability. However, as with any model with just two categories, there will be a grey area in the middle where borderline character is exhibited. This is the case for both Lewis acids and Lewis bases. Selected examples are collected in Table 3.2 below a more complete table appears later in Chapter 5. 

The FMO definition also helps explain why Pearson s hard-hard and soft-soft interactions form stable complexes. Hard compounds have a large HOMO-LUMO gap, as shown in Figure 14.9 for F. Therefore, hard Lewis acid-base complexes tend to form strongly ionic compounds, such as LiF, where the interaction is dominated by electrostatic attractions. Soft compounds, on the other hand, have a small HOMO-LUMO gap, as shown in Figure 14.9 for I, so that these types of interactions form covalently bonded acid-base adducts, where the strength of the interaction is controlled primarily by the energies of the FMOs that participate in the bonding. 

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